1. (in ecology) The interaction that occurs between two or more organisms, populations, or species that share some environmental resource when this is in short supply. Competition is an important force in evolution: plants, for example, become tall to compete for light, and animals evolve various foraging methods to compete for food. There may be a direct confrontation between competitors, as occurs between barnacles competing for space on a rock, or the numbers or fecundity of the competitors are indirectly reduced through joint dependence on limited resources. Competition occurs both between members of a species (intraspecific competition) and between different species (interspecific competition). Interspecific competition often results in the dominance of one species over another (see dominant). Since competition ultimately results in the displacement by one competitor of the others, it is to the advantage of the competitors to avoid one another wherever possible. Thus in time the competitors become separated from each other geographically or ecologically, which promotes evolutionary change. Competition for mates may lead to sexual selection.

2. (in neurophysiology) The interaction between multiple nerve endings or synapses during nervous system development that results in the elimination of superfluous connections and establishment of the mature ‘wiring’ pattern. This pruning mechanism occurs, for example, during the formation of a vertebrate skeletal neuromuscular junction. In the embryo, motor axons branch to innervate several muscle fibres, with individual fibres receiving branches from multiple neurons. However, as development proceeds after birth, this polyneuronal innervation is pruned so that each muscle fibre comes to be innervated by just a single axon branch. Similar processes prune synaptic connections in the central nervous system. One mechanism involves competition between nerve endings for a limited supply of neurotrophin from the target cells, leading to survival of some synapses and degeneration of others. Macrophage cells called glia make and break physical connections between brain cells to help maintain healthy brain function.